Dual behavior of polyelectrolyte and ionomer for an ionizable polymercontaining sulfonate groups in DMSO/THF mixtures

Zhen Tong*, Biye Ren, Feng Gao

Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, People’s Republic of China

Received 12 January 2000; received in revised form 21 March 2000; accepted 24 April 2000

Abstract

A dansyl labeled polyelectrolyte (ADDan60) was prepared by copolymerization of ionizable monomer 2-(acrylamido)-2-methylpropa-nesulphonic acid (AMPS) withN,N-dimethylacrylamide (DMAA) andN-[2–[[[5–(N,N-dimethylamino)-1-naphthalenyl]-sulfonyl]ami-no]ethyl]-2-propenamide (DANSAEP, 0.2 mol%). Reduced viscosityh sp/c, electrolytic conductivityk , and fluorescent spectrum ofADDan60 in DMSO/THF mixtures were measured as functions of the volume fractionVTHF of THF. In the region ofVTHF , 0.7, theemission maximumlem, relative intensityRF of ADDan60 in DMSO/THF to that in DMSO, and anisotropy ratior indicated that the polymermaintained its solvation layer not to change so much as the bulk solvent and exhibited a polyelectrolyte behavior, as observed fromh sp/c andk . OnceVTHF was beyond 0.70, the solvation layer was changed leading to the ion pair formation in an avalanche manner, which inducedchain aggregation and phase transition due to the dipole attraction. The results suggest a behavior transition in ionizable polymers frompolyelectrolyte into ionomer induced by medium polarity.q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Polyelectrolyte; Ionomer; Fluorescence

1. Introduction

The volume phase transition in polymer gels has beingintensively studied for about two decades since the discov-ery by Tanaka in 1980 of the hydrolyzed polyacrylamidegels swollen in water/acetone mixtures [1]. The fundamen-tal molecular force driving such a discontinuous volumechange is usually classified into four categories: van derWaals interaction, hydrophobic interaction, hydrogen bond-ing, and electrostatic interaction [2]. However, all gelsshowing the volume phase transition in response to changesin their surrounding environment contain charges on theirnetwork chains [3] and all of the reported transitions occurin aqueous solutions with only one exception [4]. Theseresults indicate that the electrostatic interaction plays themost important role in initiating the volume phase transitionof the polymer gels among the above four factors [5].

Recently Tong and Liu [6] found that highly chargedpolyelectrolyte gels of chemically crosslinked 2-(acryla-mido)-2-methylpropanesulfonic acid (AMPS) andN,N-dimethylacrylamide (DMAA) underwent the volumephase transition in a water/acetone mixture as the acetone

concentration was above a certain level. The acetoneconcentration resulting in the volume phase transitionrises only from 78 to 85 vol% when the AMPS content inthe gels increases from 8.99 to 60.76 mol%. The volumephase transition of these gels can be also induced by increas-ing the volume fraction of tetrahydrofuran (THF) in simpleorganic mixtures of dimethyl sulfoxide (DMSO) and THF[7,8]. Similarly, to increase THF in this organic mixturebeyond the above threshold can induce the precipitation ofthe corresponding linear polyelectrolytes from their homo-geneous solution of DMSO/THF [9]. These facts imply thatboth the macroscopic volume phase transition and precipi-tation are caused by the decrease in the solvent polarity. Asthe polarity is decreased, the dissociative anion on the sidechain associates with the mobile counterion bringing aboution pairs. Then, some ion pairs aggregate through thedipole–dipole attraction to form a so-called multipletamong different molecular chains or different segments ofthe same molecule as additional physical crosslinks [10,11].Therefore, the dominant interaction in the system will alterfrom the repulsive to the attractive and the polyelectrolytewill behave like an ionomer with chain associations. There-fore, the media polarity change will cause the transition inthe state of the polymers having ionizable groups.

The question arises, following what process the multiplet

Polymer 42 (2001) 143–149

0032-3861/01/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S0032-3861(00)00331-1

* Corresponding author. Fax:186-20-8711-2886.E-mail address:mcztong@scut.edu.cn (Z. Tong).

www.elsevier.nl/locate/polymer

forms in a dilute polyelectrolyte solution. Does it formgradually and individually or suddenly and synchronouslyin response to the medium polarity change? As for the situa-tion in mixed solvents, one has to take the solvent qualityinto account since the good solvent shows a higher affinityto a given polyelectrolyte. The composition of the solvationlayer surrounding the chains is not expected to be the sameas that of the bulk solvent due to the preferential absorptionof the polymer.

Fluorescence technique is considered as a powerful toolto monitor the change in the environment surrounding thechromophore for both polymer solution and gel [12–14].The chromophore attached to polymer chains can directlyreflect the situation of the microenvironment of the labeledpolymer, such as local polarity, viscosity, and segmentmobility. Early in 1975, Strauss and Vesnaver [15,16] inves-tigated the correlation between the emission spectra of thedansyl chromophore and the ionization degree of thecarboxylate group and the equilibrium in the conformationaltransition of the dansyl-labeled maleic anhydride copolymerwith alkyl vinyl ethers in solution. Chen and Morawetz [17]studied the complexation equilibrium and kinetics of theconformation transition and association of dansyl-labeledpoly(acrylic acid) (PAA) and poly(methacrylic acid)(PMA) with the polymer acting as the hydrogen bond accep-tor by measuring the fluorescence intensity of the dansylresidues attached to PAA or PMA. Horie et al. [18,19],observed the changes in microenvironment and kineticsduring the volume phase transition of dansylated polyacry-lamide (PAAm) or poly(N-isopropylacrylamide) (PNIPA)gels in mixed solvent by fluorescence spectra, anisotropy,and fluorescence lifetime. Morishima et al. [20], investi-gated micelle formation induced by hydrophobic aggrega-tion of amphiphilic polyelectrolytes containing AMPSsegments in aqueous solutions with several fluorescentmethods, including steady-state fluorescence, no-radiation-energy-transfer, fluorescence decay, etc.

In order to reveal microscopically the influence of polar-ity on the aggregation behavior of highly charged strongpolyelectrolytes, the dansyl chromophore was chemicallyincorporated into a copolymer containing AMPS similarto those used previously [9]. First, the reduced viscosityof the AMPS copolymer in DMSO/THF mixtures wasmeasured as a function of the THF volume fractionVTHF

to determine the chain conformation of polyelectrolytes.Then, the dissociation state of sulfonate groups in the

copolymer was detected by the electrolytic conductivityover the sameVTHF range. Finally, we recorded the fluores-cent spectra of the dansyl-labeled polyelectrolyte to observethe change in the chain microenvironment accompanyingthe change in the dissociation state. Thus, we get a compre-hensive understanding of the relation between the localpolarity and the aggregation transition of a polyelectrolyteinto an ionomer.

2. Experimental

2.1. Materials

Dansyl chloride (Aldrich) and 2-acrylamido-2-methyl-propanesulphonic acid (AMPS, Fluck) were used withoutfurther purification. N,N-dimethylacrylamide (DMAA,Kohjin, Japan) and acryloyl chloride (Aldrich) weredistilled before use. The silica gel powder for the separationchromatography column was of 200–400 mesh and acti-vated at 3008C for 6 h. Other reagents and solvents wereall purchased from commercial sources and purified accord-ing to standard procedures [21]. Highly pure water wasobtained through deionization and filtration with a Milliporepurification apparatus.

2.2. Preparation of dansyl labeled polyelectrolyte

Dansyl monomerN-[2–[[[5–(N,N–dimethylamino)–1–naphthalenyl]-sulfonyl]amino]-ethyl]-2-propenamide(DANSAEP) was synthesized according to the methoddescribed previously [22] (Scheme 1). The product waspurified by chromatography and characterized with1H and13C NMR spectra in CDCl3 on a Bruker DRX-400 spectro-meter (400 MHz).

The dansyl-labeled polyelectrolyte was prepared by radi-cal copolymerization of AMPS, DMAA, and DANSAEP inwater at 708C, using 0.2 mol% of ammonium persulfate asthe initiator. The total monomer concentration was0.46 mol L21 with 0.2 mol% of DANSAEP. The mixturewas stirred for 16 h under nitrogen atmosphere and thenthe product was dialyzed for two weeks against distilledwater to thoroughly remove impurities. The dialyzed solu-tion was condensed, freeze-dried and finally vacuum-driedat 608C to obtain the dansylated sample ADDan60 (yieldabout 80%). The content of 63.3 mol% AMPS in this copo-lymer was estimated from the N content determined by

Z. Tong et al./ Polymer 42 (2001) 143–149144

Scheme 1.

elemental analysis with a Heraeus CHN-O apparatus. Thecomposition and structure of this copolymer is shown inChart 1.

2.3. Measurements

The stock solution of ADDan60 in DMSO/THF mixtureswas prepared by dissolving ADDan60 in pure DMSO thenadding THF of desired solvent composition. The stocksolution was diluted with the mixed solvent of the samecomposition.

The reduced viscosity of ADDan60 in DMSO/THFmixtures was measured with a modified Ubbelohde capil-lary viscometer at 25̂ 0.058C. The flow time for mixedsolvents was longer than 100 sec.

The conductivity of ADDan60 in DMSO/THF mixtureswas measured with a immerse-type conductometer of modelDDS-11A at 25̂ 0.058C. For solutions with conductivityless than 10× 1026 Scm21 bright platinum electrodes wereused, while for solutions with conductivity higher than

10× 1026 Scm21 platinum black electrodes were used.The electrode constant was established using the standardKCl solution.

Steady-state fluorescence spectra and fluorescence aniso-tropy of the dansyl monomer DANSAEP and dansyl-labeledcopolymer ADDan60 in DMSO/THF were measured with aHitachi F-4500 fluorescence spectrophotometer at 258C.The excitation wavelength and the slit width were adjustedto 338 nm and 5 nm, respectively. The fluorescence aniso-tropy ratior is defined as [23]

r � Ik 2 I'

Ik 1 2I'

�1�

whereIk and I' are emission intensities measured paralleland perpendicular to the vertically polarized excitation,respectively.

3. Results and discussion

3.1. Reduced viscosity in DMSO/THF

According to the traditional methodology, the concentra-tion dependence of the reduced viscosityh sp/c reflects thechain conformation in the dilute solution. Fig. 1 showsh sp/cof the copolymer ADDan60 against polymer concentrationc in the DMSO/THF mixtures of different volume fractionsVTHF of THF. Similar to what Liu et al. [9] recently reportedfor the AMPS/hydropropyl methacrylate copolymer inDMSO/THF, the reduced viscosity increases with decreas-ing polymer concentration at all tested solvent composi-tions. Based on the common interpretation, this is atypical polyelectrolyte phenomenon, which is usuallyobserved in salt-free aqueous solutions, indicating that thesulfonate group is still ionized and the electrostatic repul-sion makes the chain expand in the mixtures as long asVTHF

is not beyond 0.7.Theh sp/c values at constant polymer concentrationc are

plotted as a function of solvent compositionVTHF in Fig. 2for easy comparison. It is seen that asVTHF is raised thereduced viscosity continuously decreases with a gradualincrease in the negative slope ofh sp/c vs. VTHF curves,

Z. Tong et al./ Polymer 42 (2001) 143–149 145

Fig. 1. Reduced viscosityh sp/c against ADDan60 concentrationc of solu-tion with indicatedVTHF in DMSO/THF mixtures at 258C.

Chart 1.

corresponding to the shrink of polyelectrolyte chains. WhenVTHF reaches 0.75, precipitation was observed in the concen-trated solution of the sample ADDan60. There are twocauses for this chain collapse on the addition of THF: themixed solvent becomes poorer and the dissociated polyionassociates with counterions to form ion pairs. At the presentstage, it is impossible to divide the contributions of thesetwo causes. According to the theory proposed by Khokhlovet al. [24,25], the ion-pair attraction occurs in an avalancheprocess to produce the so-called multiplet as additionalphysical crosslinkers. We consider that the fast decreasein the chain spatial size in a higherVTHF range and theprecipitation appearing in the concentrated solution aremainly induced by this positive feedback process of multi-plet formation.

3.2. Dissociation–association state transition

The association of ion pairs will cause the conductivity ofa polyelectrolyte solution to decease approximately to zero.Fig. 3 shows the electrolytic conductivityk of ADDan60 indifferent DMSO/THF mixtures varying with the molarconcentrationCAMPS of the AMPS segment in the copolymercalculated from the copolymer composition and concentra-tion. The polymer concentration is in the range of 1× 1023–0.97 gL21, lower than that used in viscosity measurement toavoid precipitation. The electrolytic conductivityk isgenerally expressed as a function of the mobile ion concen-trationCion [26,27]

k � k0 1 L0Cion 1 c �Cion� �2�wherek0 is the solvent conductivity obtained by extrapola-tion to infinite dilution, L 0 is the limiting equivalentconductivity at infinite dilution of mobile ions, and

c(Cion) represents the effect of the interionic interactionson the conductivity.

In the dilute region, the linear part can be seen at eachVTHF, while the curve bends downwards at higher AMPSsegment concentration indicating a little effect ofc (Cion).When VTHF # 0.7, k increases with the increase inCAMPS

still showing dissociation of sulfonate groups in thesemixtures. On the other hand, atVTHF� 0.8 k becomes aconstant close to the solvent conductivity independent ofCAMPS, showing no contribution tok by the sulfonate groupsin this mixture.

The realL 0 value cannot be estimated from the slope ofthe linear part ofk againstCAMPS plots in Fig. 3 because theabscissaCAMPS does not stand for the actual concentration ofmobile ions, which is depressed by the addition of THF intothe mixed solvent. From the change in the slope ofk vs.CAMPS curves, however, one can still recognize the decreasein the contribution of sulfonate groups to the solutionconductivity due to the composition change in the solvent.These results suggest that so long as the medium polarity isbelow a certain level, the polyelectrolyte would transfer fromelectrostatic repulsion dominant into ion-pair attraction domi-nant. This transition concerns the change in the selectivesolvation layer of the copolymer, which can be detected byfluorescence of our dansyl labeled polyelectrolyte.

3.3. Emission spectra of the dansyl monomer DANSAEP

As a starting stage, the emission spectra of the dansylmonomer DANSAEP were measured as a function ofVTHF

in DMSO/THF as shown in Fig. 4. The emission appearingin 493, 517 nm range blue-shifts and the intensityincreases with the increase ofVTHF in the mixed solvent.The emission is attributed to the formation of the twistedintramolecular charge-transfer (TICT) state of dansylgroups and its wavelength is referred to as the emissionmaximumlem [18,19,22]. The TICT state is related to the

Z. Tong et al./ Polymer 42 (2001) 143–149146

Fig. 2. Reduced viscosityh sp/c againstVTHF in DMSO/THF mixtures atindicated ADDan60 concentrationsc at 258C.

Fig. 3. The electrolytic conductivityk in DMSO/THF mixtures againstmolar concentrationCAMPS of AMPS segment in ADDan60 with indicatedVTHF at 258C.

torsional motion of the dimethylamino group around theamino-naphthalene bond of the dansyl residue and is stabi-lized by interactions with adjacent solvent molecules [28].The fluorescence maximum of the dansyl chromophoreexhibits a large solvachromic shift from less-polar to polarsolvents and provides useful information about its microen-vironment polarity as well as its segment mobility[18,19,22]. For example, in less-polar hexane the fluores-cence maximumlem is about 460 nm, while in water itsignificantly shifts to 530 nm [22].

As illustrated in Fig. 5,lem gradually decreases from 517in pure DMSO to 493 nm in pure THF with increasingVTHF

in DMSO/THF mixture. This blue-shift oflem manifests acontinuous decrease in the polarity of the microenvironmentaround the chromophore accompanying the mixture compo-sition. The relative intensityRF of the emission in DMSO/THF to that in DMSO is also plotted againstVTHF in Fig. 5.RF increases with an increase inVTHF over the whole compo-sition range. The results indicate that the less polar andlower viscosity medium favors the formation and stabiliza-tion of the TICT state. Chen and Morawetz [17] reportedthat with the increasing concentration of polyoxyethylene(POE-600) in aqueous solution, the emission intensity ofN-dansyl ethylene diamine was greatly enhanced together withthe blue-shift oflem due to the establishment of a morehydrophobic environment.

3.4. Emission spectra of dansylated polyelectrolyte

The TICT fluorescent emission of the dansylated poly-electrolyte ADDan60 in DMSO/THF mixtures is displayedin Fig. 6. In contrast to DANSAEP, the fluorescence inten-sity of ADDan60 decreases with increasingVTHF.

The emission maximumlem and relative intensityRF areplotted againstVTHF in Fig. 7. In pure DMSO,lem of thedansyl chromophore in ADDan60 blue-shifts to 513 nmcomparing with 517 nm for monomer DANSAEP, due tothe contribution of the hydrophobic polymer backbone tothe local polarity surrounding the chromophore on polymer

side chains. On the other hand, with increasingVTHF lem ofADDan60 blue-shifts 9 nm from pure DMSO to the mixtureof VTHF� 0.70 (504 nm), smaller than 12 nm for the dansylmonomer DANSAEP over the same solvent compositionrange (517–505 nm). Since the emission maximumlem issensitive to the local polarity, the results imply that thesolvation layer of polyelectrolyte surrounding the dansylchromophore does not vary so much as the bulk solventwhenVTHF # 0.70. The polarity of the solvation layer is asynthetic effect from the bulk solvent, the polymer back-bone, the residues on the polymer chain, and the additives inthe solution. Due to the dominant contribution of the less-polar polymer backbones, the polyelectrolyte chain iscapable of maintaining the local polarity in thesurrounding microenvironment. This argument isconfirmed by the emission maximumlem of about510 nm for the solid ADDan60 sample[29], becausethis value is very close to the plateau level in therange of VTHF # 0.70. Furthermore, whenVTHF exceeds0.70 the solvation layer is destroyed as illustrated bythe steeper decrease oflem with increasingVTHF.

Fig. 7 also demonstrates the solvent composition depen-dence of the relative emission intensityRF for the dansyllabeled polyelectrolyte ADDan60 in DMSO/THF. Incontrast to the DANSAEP,RF is always less than 1 andslightly decreases with increasingVTHF up to 0.65, then asharp decrease inRF occurs whenVTHF is above 0.70. TheRF

value is a measure of the segment mobility since the TICTstate is related to the torsional motion of the dimethylaminogroup linked to the naphthyl group [30,31]. Tazuke et al.[30,31] reported that the existence of poly(methyl metha-crylate) main chain greatly suppressed the formation of theTICT state of the 4-(N-dimethylamino)-benzoate groupbonded to poly(methyl methacrylate) side chain in ethylacetate and inn-butyl chloride dilute solutions, even thechromophore was separated from the main chain with along alkyl spacer. The abrupt decrease ofRF observed inFig. 7 at VTHF , 0.70 indicates that the TICT process is

Z. Tong et al./ Polymer 42 (2001) 143–149 147

Fig. 4. Fluorescence spectra of the dansyl monomer DANSAEP in DMSO/THF mixtures at the concentration of 1.5× 1025 molL21 at 258C.

Fig. 5. The emission maximumlem (circle) and relative emission intensityRF (square) of the dansyl monomer DANSAEP against theVTHF in DMSO/THF.

greatly hindered by the polymer chain aggregation due tothe dipole–dipole attraction of ion pairs.

Fig. 8 shows the variation of the anisotropy ratior ofDANSAEP and dansylated polyelectrolyte ADDan60 withVTHF in DMSO/THF mixtures. For the monomerDANSAEP, r remains a constant at about 0.20 over theentire range of the solvent composition. While forADDan60, r is a constant whenVTHF , 0.65, and thenincreases suddenly atVTHF� 0.70. This sharp increaseof r manifests that the relaxation motion of local chainsegments of the polyelectrolyte ADDan60 is greatlyslowed down in this solvent mixture due to the restriction ofthe ion pair attraction. This observation is consistent with thechange in the emission maximumlem and relative intensityRF reported in the above paragraphs.

The fluorescent spectra of the ADDan60 in DMSO/THFmixtures can be understood in terms of the dual phenomenaof polyelectrolyte and ionomer for an ionizable polymer[8,9,24,25]. The dissociation state of sulfonate groups onthe polymer side chain directly depend on the medium

polarity. In pure DMSO the ionization is complete, whilein the mixture ofVTHF $ 0.7 the sulfonate groups exist as ionpairs with protons. These two ionization states correspond tothe two behaviors: polyelectrolyte and ionomer. For poly-electrolytes, the electrostatic repulsion among ionizedgroups plays a dominant role and makes the polymerchain extended. On the other hand, in ionomers thedipole–dipole attraction among the ion-pairs causes thechain aggregation. Therefore, these two behaviors havethe same origin of interaction in the system, i.e. electrostaticinteraction. As observed from the change inlem, RF, andr,the polymer chain can retain its solvation layer withoutchanging much within the region ofVTHF , 0.7, resultingin the almost complete ionization of the sulfonate groups.When VTHF is beyond 0.70, the solvation layer is changedleading to the association of the sulfonate groups into ion-pairs. The dipole attraction among ion-pairs brings about thechain aggregation in an avalanche manner greatly reducingthe chain mobility. It is this polyelectrolyte–ionomer transi-tion that induces the macroscopic phase transition in poly-electrolyte gels and solutions.

Acknowledgements

The financial support to this work by the National NaturalScience Foundation of China (29725411, 29804003) isgratefully acknowledged.

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Fig. 8. The fluorescence anisotropy ratior of DANSAEP (unfilled circle)and ADDan60 (filled circle) as functions ofVTHF in DMSO/THF mixtures at258C.

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